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a North Carolina State Univ., Crop Sci. Dep., Box 7620, Raleigh, NC 27695
b E.I. du Pont de Nemours and Co., Inc., Walker's Mill, Barley Mill Plaza, P.O. Box 80038, Wilmington, DE 19880-0038
* Corresponding author (jerry_weber{at}ncsu.edu)
Received for publication August 24, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: 14C, radiolabeled carbon • CT, conventional tillage • DAT, days after treatment • DT50, field longevity (50% disappearance time) • HM, humic matter • LSA, liquid scintillation analyzer • MI, mobility index • NT, no-tillage • OM, organic matter • Rf, chromatographic reached (retardation) factor • RT, reduced tillage
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The mechanisms and driving forces of groundwater pollution under different tillage systems are still not completely understood (Flury, 1996). Using suction lysimeters in CT and NT fields, Gish et al. (1995) reported similar amounts of bromide in the percolate and evidence of preferential flow. But, twice as many detections of atrazine [6-chloro-N-ethyl-N'-methylethyl)-1,3,5-triazine-2,4-diamine] occurred under CT, whereas no differences between tillage systems occurred with alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethylacetamide)]. Gaynor et al. (1995) compared ridge tillage, NT, and conventional moldboard plowing and found a similar number of detections of atrazine and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2- methoxy-1-methylethyl)acetamide] transported to tile drainage in all tillage systems. Myers et al. (1995) studied CT and NT under simulated rainfall and found greater vertical movement of atrazine and metolachlor in NT treatments compared with CT treatments. Differences were only important after the first simulated rainfall applied 30 min after herbicide application.
Field studies done by Hall et al. (1989) showed that pan lysimeter percolate under NT fields contained substantially greater concentrations of simazine (6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine), atrazine, and metolachlor than percolate under CT systems. Herbicide losses were greatest where percolate discharge was highest, indicating the influence of macropore flow on herbicide mobility under NT fields. Isensee and coworkers (1990) monitored groundwater wells installed on CT and NT fields for 3 yr and reported that atrazine concentrations in well water during the summer months were consistently greater in the NT fields.
The lack of agreement among studies could be due to several factors. Altering soil surface management practices changes soil properties, soil chemistry, and the microclimate at the soil surface in ways that profoundly affect the behavior of pesticides (Glotfelty, 1987). Dissipation processes such as adsorption, degradation, and volatilization are also influenced by the ongoing changes in organic matter (OM) content. Other key variables are soil, air and water temperatures, moisture levels, and variation in weather patterns. Weber et al. (2006) reported that specific soil and herbicide properties influenced mobility and dissipation of atrazine, metolachlor, and primisulfuron-methyl {methyl-2[[[[[4,bis(difluoromethoxy)pyrimidinyl]amino]carbonyl]sulfonyl]benzoate} in four soils.
One approach to dealing with the inconsistencies of experimental results has been the use of leaching models. Accuracy in model predictions depends on the quality of the parameters used in modeling equations. Validity of the parameters depends on the quality of the data used to derive them. Promising results have been obtained by using the field lysimeter technique to support modeling efforts (Führ et al., 1998; Hellpointer et al., 1990; Winton and Weber, 1996). Field lysimeter studies are often conducted by applying the chemical directly to bare soil, ignoring soil surface management effects. However, we know that water usage, retention, and loss under different soil surface covers could affect soil moisture and water recharge potential. These differences in water balance dynamics create different leaching potentials of herbicides (Helling et al., 1988; Weber and Lowder, 1985).
Metolachlor is a nonionizable substituted acetamide herbicide that controls grasses and some broadleaf weeds and sedges when applied pre-emergence or preplant incorporated (Ahrens, 1994). Metolachlor has moderate aqueous solubility (Ks = 488 mg L–1 at 20°C, where Ks = aqueous solubility), moderate volatility (VP = 3.1 x 10–5 mm Hg at 25°C, where VP = vapor pressure), low to moderate soil retention [Kd = 0.1 to 2.1 mL g–1, where Kd = herbicide/soil distribution coefficient (soil retention)] (Ahrens, 1994), and moderate field longevity [DT50 = 51 d, where DT50 = field longevity (50% disappearance time)] (Miller et al., 1997). It has been ranked as having moderate to high pesticide leaching potential (Warren and Weber, 1994).
The objective of this study was to evaluate the influence of tillage, soil cover, and water input levels on the mobility and dissipation of metolachlor in field lysimeters. These results will help assess the suitability of lysimeter studies using bare surface soil for evaluating pesticide mobility and whether such studies overestimate leachability compared with different surface management alternatives.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Simulating Tillage Management and Surface Covers
Conventional tillage and RT systems were simulated as follows. For the CT treatment, the surface of the soil was thoroughly mixed to a depth of 13 cm. In the RT lysimeters, the soil remained untilled, and 10 g of chopped wheat straw mulch from baled 1990 harvest was placed on the soil surface to obtain 75% surface coverage. This equated to approximately 9600 kg ha–1 of wheat (Triticum aestivum L.) straw. ‘Tifton Green’ bermudagrass sod, soybean plants (two) (6 x 105 plants ha–1), and fallow comprised the surface cover treatments. Bermudagrass sod was transplanted on 25 May 1991 and vegetatively established by using 2-cm-thick by 20-cm-diam. discs of actively growing sod. Soybean treatments were seeded and turf established on 25 June 1991 and 6 June 1992. Fallow treatments for the CT lysimeters were hand-weeded, and the soil surface was kept bare. Bermudagrass sod and soybean were fertilized following recommendations of the North Carolina Department of Agriculture Soil and Plant Analysis Laboratory (1991).
Herbicide Treatments
Carbon-14 ring-labeled metolachlor plus Dual 8E (metolachlor, 960 g a.i. L–1 emulsifiable concentrate, Syngenta Chemical Corp., Greensboro, NC) was used to make herbicide solutions equivalent to field rates of 4.48 kg a.i. ha–1 in 1991 and 2.24 kg a.i. ha–1 in 1992. Lysimeters treated with 14C metolachlor received 0.70 MBq in 1991 (specific activity = 2.06 TBq kg–1) and 0.56 MBq (specific activity = 1.95 TBq kg–1) in 1992. Radiochemical purity of the isotope was >98.8%. Herbicide solutions were applied on 27 June in 1991 and 5 June in 1992, pre-emergent to soybean and over the top of actively growing sod. Each column received 20 mL of herbicide solution distributed in a cross-hatch pattern. Immediately after herbicide application, 123 mL of water was sprinkled over each lysimeter to simulate a rainfall event of 0.5 cm. Zero-day Dothan series soil samples from the experimental site were fortified with 14C herbicide and analyzed within 24 h.
Water Input Levels
Weather was recorded from the weather station at the Central Crops Research Station, Clayton, NC (NOAA Site no. 31–1820–07). Weekly rainfall data of the previous 10 yr [1981–1990 for 1991, 577 mm (19 L), and 1982–1991 for 1992, 644 mm (21 L)] were used to determine the amount of water to apply to the columns. If the 10-yr average was not met by ambient precipitation, water was added weekly. An additional treatment (10-yr average plus) was included each year: in 1991 [10-yr average plus 20% = 694 mm (22 L)] and in 1992 [10-yr average plus 11% = 717 mm (23 L)]. Higher water input volume at greater intensity close to zero DAT also occurred in 1992.
Lysimeter Samples Processing and Total Carbon-14 Herbicide Analyses
At 128 DAT in 1991 and 128 DAT in 1992, soil covers (plants, mulch) were harvested, sectioned into parts (roots, stem, leaves, pods, blades, and straw), bagged, labeled, and stored in a freezer at –20°C. Thereafter, plant parts and straw were oven-dried at 50°C and ground. Triplicate 0.2-g subsamples were combusted at 900°C in a R.J. Harvey OX-300 Automated Biological Oxidizer (efficiency >93%) (R.J. Harvey Instrument Co., Hillsdale, NJ). The oxidized 14CO2 was trapped in 15 mL of R.J. Harvey OX-161 Carbon-14 Cocktail. Then the 14CO2 was assayed by a Packard TRI-CARB Model 2000CA Liquid Analyzer [liquid scintillation analyzer (LSA)] (Packard Instrument Co., Downers Grove, IL). Lysimeters were extracted from the soil and sectioned every 7.6 cm by depth for a total of 12 soil samples per lysimeter. Samples were thoroughly hand-mixed and the total 14C concentration determined by combusting four subsamples in the biological oxidizer, and the 14CO2 assayed by LSA. If the combusted subsamples coefficient of variation was >20%, sections were remixed and recombusted. Leachate and runoff water were collected throughout the growing season and assayed for total 14C herbicide content by LSA. All wastes were disposed of by the North Carolina State University Life Safety Services following proper procedures (Dep. of Environ. Health and Hazardous Materials Manage., Life Safety Serv., North Carolina State Univ., 1991).
Soil Analysis for Physical and Chemical Properties
Soil samples were sent to A&L Midwest Agricultural Laboratory, Omaha, NE and to the North Carolina State Department of Agriculture Laboratory, Raleigh, NC to be analyzed for OM, humic matter (HM), particle size analysis, and pH. Organic matter was determined by chromic acid oxidation colorimetric method (Nelson and Sommers, 1982) and HM by the NaOH/DTPA-alcohol extraction method (Mehlich, 1984). Particle size analyses, for clay contents, were performed using the hydrometer method (Gee and Bauder, 1986) and soil pH (1:1 soil:water) using a glass electrode pH meter and standards.
Experimental Design and Data Analyses
Two supplemental water input levels [10-yr average (577 mm in 1991 and 644 mm in 1992) and 10-yr average plus 20% in 1991 (+115 mm) and plus 11% in 1992 (+71 mm)], two tillage systems (CT and RT), and three soil cover treatments (soybean, fallow, and bermudagrass sod) were arranged in a completely randomized factorial design with two replications each year. A mass balance assessment for each lysimeter was made by adding the 14C herbicide recovered in the soil cover (plants, mulch) and leachate to that recovered in the soil and subtracting from 100% to determine 14C herbicide that volatilized. Carbon-14 metolachlor distribution in the soil, including adding that recovered from the soil cover to the surface soil fraction, and that recovered in the leachate to the 84- to 91-cm depth fraction and normalized to 100% recovered in the soil, was used to calculate mobility indices (MI) and retardation factor (Rf) indices, as described by Weber et al. (1999), using MI = 
D x F, where D = mean depth in cm and F = fraction of chemical present, and Rf = MI/MImax. Leachate volume from each lysimeter was measured weekly and totaled at 128 DAT. The measured 14C distribution data were subjected to analysis of variance (ANOVA) contrasting water input, tillage, soil cover, replication, and year effects (SAS Inst., 1988).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Water Input, Tillage, and Surface Cover Main Effects
Year effects were significant (
= 0.05) between the 1991 and 1992 field studies; thus, the measured data from each year were analyzed separately. Differences were likely due to the 7% increase in water input and greater rainfall observed in 1992 that occurred near treatment initiation. Tillage, soil cover, and tillage–soil cover interactions affected 14C distribution of 14C metolachlor. Measured parameters were averaged across water input treatments each year to make comparisons (Tables 1 to 4). Main effects of tillage, soil cover, and year are compared in Tables 1 and 2 while interaction effects of tillage and soil cover (tillage–soil cover) and year are compared in Tables 3 and 4.
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Twice as much 14C was recovered in RT soil cover than in CT soil cover in 1991, probably because the added straw in RT retained significant amounts of the herbicide (Table 1). However, equal and lesser amounts were recovered in the two tillage treatments in 1992. The lack of tillage effect and the lesser amounts recovered in the 1992 soil cover may have been the result of the greater rainfall in the first 15 DAT in 1992 (340 mm) than in 1991 (140 mm), which may have leached the herbicide, as was previously reported (Lowder and Weber, 1979). Herbicide removal from crop residues by rainfall has been reported to depend on intensity, quantity, and time after herbicide application (Dao, 1995; Lowder and Weber, 1979; Myers et al., 1995; Weber and Lowder, 1985).
Seven percent more 14C was recovered in soil under RT than under CT in 1991 (Table 1). Total 14C recovery was also higher in subsoil and leachate. Similar amounts were recovered in surface soil of RT and CT treatments under the greater water input in 1992, and 7 to 15% lesser but equal amounts of 14C were recovered in subsoil of these treatments. Carbon-14 in leachate under RT was twice that under CT in each of the 2 yr but was five times greater in 1992 than in 1991 due to the greater water input (Table 1).
Total leachate volumes collected under the two tillage systems were similar both years (Table 1) even though there were different water input patterns and greater input occurred in 1992, suggesting that evapotranspiration was similar under the two systems and 14C recovered and quantity of surface runoff were similar and negligible both years (data not shown). Dao (1995) reported that crop residues can intercept herbicide applications and mitigate herbicide leaching in soil. Also, Sadeghi and Isensee (1992) observed that different tillage systems modified atrazine distribution in soil. Green et al. (1995) reported that high levels of residue cover could enhance preferential flow of atrazine in medium- to high-conductivity soils, which may also have contributed to the greater mobility (Rf) of metolachlor under RT than under CT in 1991. In 1992, all of the measured parameters indicated that 14C metolachlor was of equal mobility under RT and CT conditions, with the exception of 14C recovered in the leachate where five times higher levels of 14C were recovered under RT than under CT than in 1991 (Table 1). This coupled with the higher Rf under RT suggests greater metolachlor mobility under RT than under CT. Greater water input in 1992 than in 1991 and greater volatilization losses in 1992 probably reduced the tillage effects, as evidenced by the near double amount of leachate passing through the lysimeters in 1992.
Soil cover main effects on 14C metolachlor volatilization indicated that greater volatilization of the herbicide occurred from bermudagrass sod than from fallow or soybean treatments, which had equal losses in 1991 (Table 2). Equal amounts of 14C metolachlor volatilized from the three soil cover systems in 1992.
Carbon-14 recovered from soil cover in 1991 indicated that sod intercepted three times more 14C metolachlor in 1991 and seven times more in 1992 than soybean treatments. Soybean treatments intercepted five times more than fallow treatments in 1991 and 12 times more in 1992 (Table 2).
Greater interception of metolachlor by plants and/or mulch resulted in greater amounts recovered in fallow surface soil than under soybean or sod treatments, respectively, in 1991 (Table 2). Equal amounts of the herbicide were recovered in surface soil under fallow and soybean in 1992. Both treatments had twice the 14C as that recovered under sod. Total soil 14C and subsoil 14C recovered under fallow was equal to that recovered under soybean, which was 17% greater in 1991 and 9% greater in 1992 than that recovered under sod in the 2 yr (Table 2). Two to five times as much 14C was recovered in leachate under fallow than was recovered under soybean or sod (Table 2).
The range of possible MI values was 3.8 to 87.6 while the range of Rf values was 0.04 to 1.0. The higher the number (MI or Rf value), the greater the mobility of the herbicide. Calculated 14C soil mobility (Rf) of metolachlor was greater under fallow than under soybean, which was greater than under sod in 1991 (Table 2). In 1992, 14C metolachlor soil mobility (Rf) was fallow = soybean > sod.
Total leachate volume recovered under fallow was greater than that recovered under soybean or sod in 1991. However, in 1992, similar quantities of leachate were recovered under all three treatments (Table 2). The leachate volumes in 1992 were twice those found in 1991 due to the greater water input.
In most cases, 14C distribution in the lysimeters suggests that the order of decreasing mobility of 14C metolachlor was fallow > soybean > sod (Table 2). Retention of herbicides by sod thatch (Lickfeldt and Branham, 1995) and crop residues (Green et al., 1995; Lowder and Weber, 1979) and reduced mobility of metolachlor under soybean compared with fallow (Keller and Weber, 1997) due to reduced soil water available for leaching probably all contributed.
Tillage–Soil Cover Interaction Effects
Carbon-14 metolachlor distribution and dissipation, as influenced by tillage and sod cover at 128 DAT, are presented in Tables 3 and 4. Carbon-14 metolachlor volatilization from the soil ranged from 20 to 43% of applied during the 2-yr study (Table 3). Carbon-14 volatilization from sod was equal to volatilization under CT-soybean and CT-fallow (30 to 40%). Conventional tillage-soybean and CT-fallow had greater volatility losses (30 to 33%) than RT-fallow and RT-soybean (20 to 24%) in 1991. However, volatilization losses in 1992 were 10 to 20% greater than 1991 and showed no tillage–soil cover differences. As mentioned previously, this yearly difference may have resulted from the greater water input and higher soil moisture in 1992, which has been reported to increase pesticide volatilization (Glotfelty, 1987; Weber et al., 2002). Keller and Weber (1997) reported 33 to 58% 14C metolachlor volatilization each year from lysimeters in three 1-yr studies with greater volatilization occurring under CT-soybean than under CT-fallow 60% of the time in measurements made 30, 60, 90, and 120 DAT.
Carbon-14 recovered in soil cover was greatest in sod treatments (15 to 17%), followed by RT-soybean (2 to 7%), CT-soybean (2 to 4%), RT-fallow (0.3 to 2%), and CT-fallow (0%) both years (Table 3). Carbon-14 recovered in soybean plants on CT-soybean treatments was in general agreement with amounts recovered by Keller and Weber (1997) from similar treatments where it ranged from 0.8 to 1.7% at 120 DAT.
Surface soil 14C metolachlor ranged from 12 to 20% under RT and CT treatments both years and in lower amounts (8 to 13%) under sod (Table 3), due to the greater interception by the sod as reported by Lickfeldt and Branham (1995). Subsoil 14C metolachlor was present in greatest amounts under RT and CT treatments and ranged from 44 to 58% in 1991 to 39 to 42% in 1992. Carbon-14 metolachlor was found in lesser amounts under sod (34 to 36%) both years (Table 3).Total 14C recovered from the soil in 1991 was greatest under RT treatments (71 to 72%), followed by CT treatments (63 to 68%) and then bermudagrass sod (49%). Total 14C metolachlor recovered was inversely related to losses through volatilization and/or soil cover interception (Table 3). In 1992, similar amounts (51 to 55%) of 14C were present in the soil under RT and CT treatments, which were 10% greater than under bermudagrass sod (42%).
The decreasing order of 14C in leachate was RT-fallow > CT-fallow > RT-soybean = bermudagrass sod = CT-soybean but was four times greater under fallow, 10 times greater under soybean, and 24 times greater under sod in l992 than in 1991 (Table 3).
Greatest leachate volume collected in 1991 was 4.7 L (23% of input water) under the CT-fallow treatment, followed by RT-fallow, RT-soybean, bermudagrass sod, and CT-soybean (Table 3). Relatively similar amounts of leachate (mean = 7.6 L) were collected in 1992 from all treatments under the 7% greater water input, which amounted to 35% of the input water for the year.
Carbon-14 distribution in the Dothan subsoil ranged from 0.2 to 1.8% at the 83- to 91-cm depth to 11 to 18% at the 8- to 15-cm depth in 1991 and from 0.9 to 1.6% at 83 to 91 cm to 5.2 to 11% at the 8- to 15-cm depth in 1992 (Table 4). Carbon-14 metolachlor mobility (Rf) was RT-fallow = CT-fallow > RT-soybean = CT-soybean > bermudagrass sod both years. Mobility was 23 to 59% greater in 1992 than in 1991 due to the 7% greater water input for that year.
Although field lysimeters maximize vertical movement of water and modify and/or reduce lateral movement, the study illustrates that tillage and soil cover influence the mobility of metolachlor through soils and suggests that the study of herbicides through fallow soils overestimates the movement of pesticides that are used in crop production.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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